Penises, as a general rule, are some of the more improbable structures in biology (especially bird penises). There are many ways in which they are marvels of engineering—and prime examples of the truly weird avenues evolution will explore, as long as more babies result. One major miracle is that they manage to stand up, something achieved, in most penises you’re likely to be familiar with, with a huge rush of blood. But bird penises (of course! showoffs) have taken another route. They stand up with lymph instead.

Lymph, if you don’t recall, is blood’s often-overlooked sibling. Charged with aiding in cleaning out the body’s waste and shuttling around various immune components, it circulates in a system of vessels similar to blood vessels (albeit more slowly, since the system’s not hooked up to a central pump) and contains a rich mixture of immune cells, metabolites, and other goodies. Scientists have known for a long time that most bird penises use lymph to get their pick-me-up, but one group of birds had never had their gear fully examined: the ratites, which include ostriches and emus. This gaping hole in our knowledge had languished for a distressingly long time.

Now, however, through four dissections and “direct manipulation of fresh penises,” ornithologists Patricia Brennan and Richard Prum have confirmed what several biologists starting in the 1830s speculated on, but never fully explored: ostriches and emus do it like other birds. The researchers found that the penis’s base is padded with lots of spongy tissue used specifically to make lymph and that plenty of lymph vessels were present. The finding is evolutionarily relevant: it confirms that having a lymph-filled penis really is something that all penis-endowed birds share, rather than a trait developed after ratites and other birds began to diverge.

An erect ostrich penis.

For the researchers, however, people’s interest in ostrich penises has been more surprising than the results of the study. “It’s been a bit of surprise to us. Scientifically, this was a pretty sleepy study. Just cleaning up some 19th century sloppiness,” Prum told me in an email. “But there has been a whirlwind of excitement!” Of course, Rick! We always rise to the occasion.

Is that what the ostrich penis normally looks like, or is that an insicion leaking lymph? It’s location off to one side just seems odd…

http://twitter.com/vero_greenwood Veronique Greenwood

Here’s what Prum says about that flow: “It’s the “sulcus”. Just a groove out of which the semen flows. Only
mammals have an enclosed urethra (where the penis grows around to
seal at the bottom to form a closed external tube. Reptile penises [and birds]
just have a sulcus.”

Basically, the semen comes out of that hole and flows down the penis (you’ll notice the groove going down its length). Works better as a semen delivery system if the penis is in something. That way, the vaginal wall provides the other half of the tube.